The present invention relates to a rotary screw machine for a gaseous working medium with at least one screw rotor operating in a casing including a low and a high pressure end section, which rotor at operation is exposed to axial gas forces in a direction from the high pressure end section towards the low pressure end section, defined as the positive axial direction. The rotor has at least one shaft journal mounted in beatings in one of said end sections, which beatings include thrust bearing means with main thrust bearing means adjacent the rotor and balancing thrust bearing means adjacent the outer end of the shaft journal, which thrust bearing means are provided with thrust balancing means located axially between the main and balancing thrust bearing means and exerting a positively directed axial force on the main thrust bearing means and a negatively directed axial force on the balancing thrust beating means.
The term thrust bearing in this application is used to mean bearings which transfer at least axial loads, i.e. pure axial beatings as well as bearings transferring both axial and radial loads.
A rotary screw machine with such a thrust balancing arrangement is disclosed in U.S. Pat. No. 4,915,514, which hereby is incorporated by reference. In that disclosure the general advantages gained by a thrust balancing device are discussed as well as the particular advantages attained by the above specified kind of thrust balancing, namely longer running life for the thrust bearings and a simplified exchange of bearings.
In that known device, the main thrust beating means transfer axial forces in the positive direction only whereas the balancing thrust beating means transfer axial forces in both directions. A preloading spring acts on the balancing thrust bearing means, securing a proper axial localisation of the rotor so that a certain play between the high pressure end of the rotor and the adjacent end wall is assured when the machine is at rest or idles. At operation the positively directed axial gas Forces on the rotor are partly balanced by a piston of the thrust balancing means acting on the outer ring of the balancing thrust bearing means in the negative axial direction. The balancing force is mainly received in that gas from the high pressure side of the machine is conducted to the piston to act thereon. The balancing force will be larger than the force from the preloading spring with the effect that the resulting force on the outer ring of the balancing thrust bearing means is negatively directed, which force is transmitted to the rotor and counteracts the axial gas forces. The main thrust bearing means therefore need to transfer only the remaining part of the axial gas forces.
This known device functions satisfactory for applications where the discharge pressure (when the machine is a compressor) is above a certain level, in which case it is possible to dimension the piston large enough to attain a force which is larger than the preload force. The available space for the piston, however, is limited, which in cases when the discharge pressure is not so high will raise problems to attain a balancing force exceeding the preloading force, which has to be above a certain level. In such cases, e.g. when the machine is a refrigeration compressor using refrigerant of the type R134a, the known device therefore cannot function satisfactory.